Another vote for the soldering iron - and tapping it with a hammer too.

Heat it up, jar it loose.

And of course use a flat-ground screwdriver that fits the slot exactly.

Sometimes I use Kroil, letting it soak in overnight.

I used to have a security outfit bring me a shoe-box full of .38 snubbies to look over once a year, whether they needed it or not. - Every once in a while I would run across a stubborn screw like that.